Image artifacts resulting from motion have been important issues of research in many medical imaging modalities because they may degrade the image quality and cause inaccurate clinical interpretation of images. Artifacts can arise when an object being imaged (sample) is moved during data acquisition but is assumed stationary in the image reconstruction process. In each imaging modality, motion artifacts can be present in different forms and with different magnitudes. Understanding basic motion effects in a particular imaging method is an essential step toward the development of techniques to avoid or compensate resulting artifacts. Optical interferometric imaging methods using frequency domain ranging have recently received considerable interest due to their high image acquisition speed and sensitivity.
Two frequency domain techniques have been demonstrated: spectral-domain optical coherence tomography (SD-OCT) as described in A. F. Fercher et al., “Measurements of intraocular distances by backscattering spectral interferometry,” Opt. Comm. 117, 43-48 (1995), G. Hausler et al., “Coherence radar and spectral radar—new tools for dermatological diagnosis,” J. Biomed. Opt. 3, 21-31 (1998), M. Wojtkowski et al., “Real time in vivo imaging by high-speed spectral optical coherence tomography,” Opt. Lett. 28, 1745-1747 (2003), N. Nassif et al., “In-vivo human retinal imaging by ultra high-speed spectral domain optical coherence tomography,” Opt. Lett. 29, 480-482 (2004), S. H. Yun et al., “High-speed spectral domain optical coherence tomography at 1.3 μm wavelength,” Opt. Express 11, 3598-3604 (2003), and optical frequency domain imaging (“OFDI”) S. R. Chinn, E. Swanson, and J. G. Fujimoto, “Optical coherence tomography using a frequency-tunable optical source,” Opt. Lett. 22, 340-342 (1997), B. Golubovic et al., “Optical frequency-domain reflectometry using rapid wavelength tuning of a Cr4+:forsterite laser,” Opt. Lett. 22, 1704-1706 (1997), F. Lexer et al., “Wavelength-tuning interferometry of intraocular distances,” Appl. Opt. 36, 6548-6553 (1997), S. H. Yun et al, “High-speed optical frequency-domain imaging,” Opt. Express 11, 2953-2963 (2003), the entire disclosures of all of which are incorporated herein by reference. Using the SD-OCT technique, the spectral interference fringe can be measured in the spatial domain by means of a diffraction grating and a charge-coupled device (“CCD”) array. In exemplary OFDI techniques, the spectral fringe is mapped to the time domain by use of a frequency-swept light source and measured with a photodetector as a function of time. In both methods; axial reflectance profile (A-line) is obtained by performing a discrete Fourier transform of the acquired data. Since the Fourier transform process involves integration of the entire data set obtained in single A-line period, the signal-to-noise ratio (“SNR”) is enhanced relative to time domain ranging, as described in S. H. Yun et al., “High-speed optical frequency-domain imaging,” Opt. Express 11, 2953-2963 (2003), R. Leitgeb, et al. “Performance of Fourier domain vs. time domain optical coherence tomography,” Opt. Express 11, 889-894 (2003), J. F. de Boer et al., “Improved signal-to-noise ratio in spectral-domain compared with time-domain optical coherence tomography,” Opt. Lett. 28, 2067-2069 (2003), and M. A. Choma et al., “Sensitivity advantage of swept source and Fourier domain optical coherence tomography,” Opt. Express 11, 2183-2189 (2003), the entire disclosures of all of which are incorporated herein by reference This improvement in SNR is particularly advantageous for applications requiring high image acquisition rates such as screening for disease and surveillance of large tissue volumes. It is, however, possible that the integration effect enhances the sensitivity to sample motion because the motion-induced change in signal is also integrated over the entire A-line acquisition period.
Spectral-domain optical coherence tomography (“SD-OCT”) makes use of low-coherence spectral interferometry to obtain cross-sectional images of a biological sample. Interference fringes as a function of wavelength are measured using a broadband light source and a spectrometer based on a charge-coupled-device (“CCD”) camera. The axial reflectivity profile of a sample, or an A-line, can be obtained by a discrete Fourier transform of the camera readout data. This imaging technique has recently gone through rapid technical development to demonstrate high quality imaging of biological samples with fast image acquisition time, an order of magnitude faster than state-of-the-art time-domain OCT systems. The recent advancement in imaging speed may lead to the utilization of SD-OCT in a number of clinical applications in the near future.
The SD-OCT systems that have been used to date utilized either a continuous-wave (“cw”) broad-spectrum light source, such as super luminescent diodes (“SLD”), or ultrashort mode-locked pulses with a high repetition rate in the range of 10-100 MHz. In both cases, the CCD array is generally illuminated constantly, and therefore the exposure time of the CCD camera determines the signal acquisition time for a single A-line. In this case, a path length change in the interferometer during image acquisition results in phase drift in the interference fringe. If the phase drifts over more than μ during a single A-line acquisition, the interference fringe can be completely erased, resulting in a degradation of SNR. This motion artifact can be caused by axial motion of a sample relative to the probe beam. By comparison, transverse sample motion or transverse beam scanning does not result in fringe washout. However, the transverse motion can result in degradation in transverse resolution and SNR. In medical imaging in vivo, the motion effects can arise from various sources. The main causes include patient motion, physiological phenomena such as cardiac motion, blood flow, pulsation, and catheter movement associated with beam scanning or uncontrolled movement of operator's hand. Furthermore, environmental changes such as mechanical vibration, sound waves, and temperature drift can alter the path length difference in the interferometer, resulting in SNR degradation through fringe washout. Considering that cameras appropriate for SD-OCT typically provide exposures times longer than 10 μs, a solution to the fringe washout problem will be required for biomedical applications where sample and probe motion is common.
Therefore, one of the objects of the present invention is to reduce or eliminate the motion artifacts.